Heart palpitations can be caused by: * Exercise * Anxiety, stress, fear * Fever * Caffeine, nicotine, cocaine, diet pills * Overactive thyroid * Anemia * Hyperventilation * Low levels of oxygen in your blood * Medications such as thyroid pills, asthma drugs, beta blockers, or anti-arrhythmics. (Medications to treat an irregular heart rhythm will sometimes cause a different irregular rhythm). * Mitral valve prolapse, a condition in which the valve that separates the left upper chamber (atrium) from the left lower chamber (ventricle) of the heart does not close properly * Heart disease